Many industrial processes involve low grade cooling of fluids by rejecting heat from a hot fluid into a sink under conditions where the temperature difference is too small to justify using waste heat converters for producing electricity. Steam condensers for steam turbine power plants, industrial processes involving distillation operations and producing chemicals, are examples in which low grade heat is rejected into ambient air or water. The temperature difference between the fluid to be cooled and the temperature of the heat sink is often so small that recovering heat in the fluid is too costly in terms of the size and expense of a suitable heat exchanger.
In industrial processes of the type described above, air cooling is often utilized sometimes due to a lack of available water, such as sea, lake, or river water, but often to minimize the environmental impact of water cooling. For example, cooling towers are conventionally used in conjunction with water cooled power plant condensers in order to conserve water, and these towers input water from the system into the atmosphere in the form a vapor plume that contributes to air pollution. Furthermore, such installations are noisy and can cause visibility problems particularly the cooling towers are located in the vicinity of roads or highways. Cooling towers further require periodic blow-down to extract undesirable chemicals from the cooling system and, often, the cooling water requires treatment, for example the addition of certain chemicals to the water. Disposal of these chemicals is often an environmental problem.
Frequently, cooling ponds are required which also have their impact on the environment. In addition, cooling towers require large quantities of relatively quality water.
Having selected air cooling, the designer is faced with both cost and operational problems. The cost problems are best appreciated from considering how relatively small temperature differences affect heat transfer. In engineering terms, small temperature differences translates into a requirement for a large heat transfer surface areas. Traditionally, for process cooling, this has resulted in using large numbers of finned tubes through which the hot fluid flows; and this is costly in terms of material and fabrication. Operational problems are concerned with parasitic losses due to the energy required to force large masses of air through the finned tubes, and the energy required to pump the fluid to be cooled at a relatively high velocity through long length, small diameter, finned tubes in order to effect heat transfer. For steam condensation in power plant operations, for example, parasitic losses also arise from the necessity for maintaining a large vacuum in the air-cooled condenser which implies the circulation of relatively large volumes of steam in the condenser. To reduce these parasitic losses, the diameter of the finned tubes can be increased and their length shortened, but this approach increases construction costs and reduces heat transfer efficiency. Furthermore, fans are required which consume electrical power that must be supplied form an external source.
Due to the relatively large heat exchange area and piping needed when air cooling is utilized, indirect air cooling has been used where an additional cooling cycle is introduced. The fluid flowing in the additional cooling cycle cools the hot fluid. Heat is extracted from the additional cooling fluid by passing the fluid through heat exchangers positioned in a conventional natural draft dry cooling tower. By using water as the cooling fluid in the additional cooling cycle, a direct contact condenser has been used in steam operated power plants. Experience reported in ASME paper 90 JPGC/PWR-25, however, shows that the anticipated advantages concerning the efficiency, compared with a conventional surface condenser, could not be achieved. In addition, the increased quantity of water brought about by the use of the additional cooling cycle caused problems with the main cycle. As disclosed in the ASME paper, the alternative solution was a surface indirect condenser used for condensing steam exiting the condenser. In both of these approaches, substantial additional equipment had to be used including a dry, natural draft, cooling tower.
There are other problems in industrial, or other processes involving air cooling a hot fluid that is "dirty" i.e., contains foreign material that fouls the interiors of the finned tubes. The large number and the long length of these tubes often present formidable cleaning problems. In air-cooled steam condensers, the large number of tubes and the potential for leakage at their header connections represent a further problem.
It is therefore an object of the present invention to provide a new and approved method of and apparatus for cooling hot fluids which substantially overcomes or significantly reduces the problems outlined above.